The popup showed no title when hovering the icon of the running Thunar or xfce4-terminal.

I tried „pin application“ via right click on Thunar or the xfce4-terminal and then closed it.

I could not restart the program by clicking the icon. Nothing happened.

To fix this, just remove or rename these files depending on the program you are using:sudo mv /usr/share/xubuntu/applications/Thunar.desktop /usr/share/xubuntu/applications/Thunar.desktop.backupsudo mv /usr/share/xubuntu/applications/xfce4-terminal.desktop /usr/share/xubuntu/applications/xfce4-terminal.desktop.backup
Do the same for debian-xterm or debian-uxterm if you have problems with these applications and DockbarX.
Afterwards, reload the Xubuntu window manager:xfwm4 --replace &